The invention relates to a process for producing a continuously cast primary product, in particular broad slabs, having a thickness of the primary product D>100 mm and a width of the primary product B=2700 mm to 3500 mm at a casting rate vc<2 m/min in a continuous-casting plant. In the process molten material, preferably molten steel, from a reservoir is introduced via a submerged nozzle into a permanent mold, which is formed by wide side walls and narrow side walls. The primary product has partially solidified in the permanent mold and has a liquid core and a solidified strand shell. It is continuously withdrawn from the permanent mold and cooled. The invention also relates to a continuous-casting plant for producing a continuously cast primary product and to a submerged nozzle for use in this continuous-casting plant.
When the submerged-casting process is used in continuous casting, it is customary for the molten material to be introduced from a reservoir, generally a tundish, through a submerged nozzle which is coupled thereto, into an oscillating permanent mold below a bath level which is covered with casting powder. This procedure can be carried out without problems for small permanent-mold cross sections, but, in particular with permanent molds with a high width:thickness ratio, leads to difficulties with the formation of an optimum permanent-mold flow and therefore impairs uniform strand shell growth during the gradual solidification of the molten material at the cooled permanent-mold wall.
DE-C 197 24 232 has already disclosed a process for producing primary products in the form of slabs in a continuous-casting plant using the principle described above. In this case, the molten material is introduced into the permanent mold through a submerged nozzle which lies below the bath level, is open at the bottom in the casting direction and widens out in the shape of a funnel toward the narrow side walls of the permanent mold. If the dimension rule given in claim 2 of DE-C 197 24 232 for the submerged nozzle in terms of its width (b) is applied to the widths (B) of the primary product or of the permanent mold of from 2700 mm to 3500 mm which are provided for according to the invention, the result is submerged nozzle widths (b) of approximately 385 mm to 2250 mm, which cannot be produced from refractory materials with the durability when used at high temperatures which is required for long-term operation. Moreover, such wide submerged nozzles exacerbate the known problems with the gap flow between submerged-nozzle wall and wide side wall of the permanent mold.
DE-C 196 47 363 has disclosed a submerged nozzle which is suitable for use for the continuous casting of slabs and in which the molten material emerges below the bath level through outlet openings, which lie laterally opposite one another, toward the narrow side walls of the permanent mold. An essential feature of this submerged nozzle is the constant distance between its outer wall and the strand shell which is formed along the wide side wall. This means that this submerged nozzle is suitable for a width:thickness ratio of the cast strand or of the permanent-mold cross section of at most 8. However, if the width:thickness ratios are higher, this submerged nozzle cannot ensure a permanent-mold flow which creates uniform solidification conditions.
During the continuous casting of primary products with high widths, despite standard casting rates of 1.0 m/min to 1.2 m/min, very high steel throughputs of up to 4 to/min and above are reached. In practice, it has been found with these high steel throughputs that the swirl-forming flows which form in the permanent mold are very unstable. The guidance property of the permanent-mold chamber for this flow deteriorates at increasing distance between the submerged nozzle outlet opening and the narrow side wall. In addition, the emerging jet, on account of its high local flow velocity in the submerged nozzle and immediately after it emerges from the submerged-nozzle outlet opening, on account of the resistance of the molten material and the high wall friction along the permanent mold walls, is greatly decelerated and, on account of the reduced pressure between casting level and emerging jet, is diverted upward toward the casting level. A fluctuating, oscillating bath movement, which has an adverse effect on the product quality, is visually observed.
This unfavorable formation of the flow conditions is illustrated in FIG. 1 on the basis of a filament of flow. In the region between the submerged nozzle and the permanent-mold narrow side wall, the emerging jet hits the bath surface, where it splits into two partial jets. This phenomenon leads to a lack of uniformity in the melting of the casting powder at the bath surface and to local adverse effects on the sliding characteristic between stand and permanent-mold wall. When conventional submerged nozzles are used to cast broad slabs, it is difficult for the above-mentioned reasons, to produce a favorable and stable flow in the permanent mold.